1. Field of the Invention
This disclosure relates to sensing variations in velocity such as in a motor vehicle, and more particularly to a method and structure for mounting an accelerometer on a support.
2. Description of the Prior Art
Micro-machined chip accelerometers are well known in the art. Examples are disclosed in U.S. Pat. No. 5,060,504 issued Oct. 29, 1991 to White, et al., U.S. Pat. No. 5,221,400 issued Jun. 22, 1993 to Staller, and U.S. Pat. No. 5,233,874 issued Aug. 10, 1993 to Putty et al. As is well known, an accelerometer is a device which measures acceleration, or more accurately measures force exerted by a body as a result of a change in the velocity of the body. A moving body possesses an inertia which tends to resist the change in velocity. It is this resistance to any change in velocity that is the source of the force exerted by the moving body. This force is directly proportional to the acceleration component in the direction of movement when the moving body is accelerated.
In a typical micro-machined accelerometer formed of silicon (a "chip"), a central, typically spherical or rectangular shaped mass is suspended by one or more microbridges. The bridges are attached to a supporting substrate which circumscribes the mass, with a gap provided therebetween. The mass is supported within and has free movement relative to the supporting substrate. The individual micro-bridges within e.g. each pair of micro-bridges are positioned at opposing edges of the mass such that the pair's longitudinal axis constitutes a common axis across the surface of the mass.
The movement of the mass is measured, for instance, by measuring a corresponding change in the output of a Wheatstone bridge incorporating beam piezo-resistors formed in the micro-bridges.
Typically such micro-machined silicon chip accelerometers require external circuitry to process the signal output by the accelerometer, for instance for triggering an automobile air bag deployment system. Such accelerometers hence are commonly used in automobiles and other vehicles.
Accelerometers are constrained in that typically a micro-machined accelerometer as described above has a single axis sensitive to acceleration. That is, it can only measure acceleration along a line perpendicular to a plane defined by the principal surface of the chip. The principal surface of the chip is in the plane of the chips, from which side during fabrication various fabrication steps (masking, etching, etc.) are performed. For an automobile airbag system the direction of acceleration which must be sensed in the event of a collision is typically along a line lying in a horizontal plane (parallel to the ground).
A typical prior art structure for mounting such a micro-machined accelerometer chip is shown in a front view in FIG. 1a. Here a conventional metal "can" chip package 12 defines a central recess 10 in which are conventionally mounted the actual accelerometer chip 14 and the associated electronic circuitry, here present in ASIC circuit 16. ASIC means Applications Specific Integrated Circuit which is typically used, but other types of integrated circuit will also perform this function. It is to be understood that in some such accelerometers, this circuitry is integrated on the accelerometer chip itself.
The accelerometer chip 14 is electrically connected to the ASIC chip 16 by conductors 18 carrying electrical signals between the two chips 14, 16. The metal can 12 is sealed by a lid (not shown), while the accelerometer chip 14 and ASIC chip 16 are held on a ceramic substrate 20 fixed inside can 12. Also provided are screws 24a, 24b (and corresponding holes not shown) for securing can 12 to a mounting bracket (described below).
The electrical conductive leads which connect chips 14 and 16 to the remainder of the system (the actual connection is not shown for simplicity here) terminate in this case at conventional edge clip terminations (pins) 28a, 28b, etc. These electrically connect to printed circuit board (PCB) 36 held by standoffs 38a, 38b on the floor of metal housing 40.
To better illustrate the structure of FIG. 1A, a side view along line A--A of FIG. 1a is shown in FIG. 1b. The lid which seals (by a weld) can 12 (omitted from FIG. 1a) is designated by reference number 26. FIG. 1b also illustrates the support structure for the can 12, which is mounted on a metal base plate 28 with glass to metal seals. Base plate 28 in turn is attached by screws 24a, 24b to a mounting bracket 34. Mounting bracket 34 is a robust structure formed for instance of metal, plastic or ceramic. The purpose of mounting bracket 34 is to properly mount can 12 in a vertical orientation on the sidewall of housing 40. Bracket 34 is held onto the sidewall by screws, rivets, or other means (not shown). The direction of acceleration is as shown by the arrow labelled "AXIS of ACCEL". Thus the sole purpose of base plate 28 and mounting bracket 34 is to vertically mount can 12 so that it is perpendicular to PCB 36. Typically in a vehicle, PCB 36 is thereby installed in a plane parallel to that of the surface of the earth.
As shown, lead 28a (and the other leads 28b, . . . , 28g) extend from behind metal can 12 (detail not shown for simplicity) to connect to electrical connectors on PCB 36. PCB 36 provides the connections to the actual air bag firing device, typically housed in the steering wheel, whereas the housing 40 is typically mounted under the dash board or on the floor board of the vehicle. The structure of FIGS. 1a and 1b functions satisfactorily. However it has the major disadvantages of being relatively bulky and expensive due to the rather elaborate mounting structure required to rigidly mount the accelerometer chip in the desired orientation relative to 1) its axis of sensitivity; and 2) the horizontally mounted PCB 36. Also, the relative complexity of the mechanical structure and leads 28 cause reliability problems.
Analog Devices has disclosed (Electronic Design, Aug. 8, 1991, "Airbags Boom When IC Accelerometer Sees 50G" see FIG. 5) a micro-machined accelerometer chip whose mass moves in the plane of the chip. This chip allows the axis of sensitivity to be parallel to the expected acceleration phenomenon. However, this is a special accelerometer chip apparently available only from this single supplier.
Thus it would be highly desirable to reduce the cost and increase reliability of mounting accelerometer chips in an automobile, other vehicle, or other application without the need for an expensive and bulky mounting structure, and using a standard accelerometer chip.